Processes for the preparation of high-purity silicon have long been known. DE 10 2004 027 563.7 discloses, for example, an energy and cost-efficient preparation process for high-purity silicon wherein a monosilane/hydrogen gas mixture is thermally decomposed and there results in the gas phase powdered silicon which is then mechanically compacted. In this preparation process, it is possible for the silicon which has separated out of the gas phase to be deposited as a layer on the heated inner wall of the reactor vessel. The reactor vessel consists generally of quartz glass having a different coefficient of thermal expansion from silicon. During operation of the reactor, increasing amounts of silicon are deposited on the inner wall of the reactor vessel. As a result, on the one hand, the conduction of heat from the heating device arranged outside the reaction chamber to the reaction chamber decreases and, on the other hand, after a certain period of time has elapsed the reactor vessel has to be mechanically or chemically cleansed of the deposited silicon layer. This may interrupt the operation of the reactor. In addition, the differing coefficients of thermal expansion of the reactor vessel and the silicon layer result, on cooling of the reactor vessel, in substantial forces and tensions between the deposited silicon layer and the quartz glass. This can lead to damage to the reactor vessel, in particular to tears and cracks which pass into and contaminate the deposited powdered silicon.